This invention relates to a high efficiency optical semiconductor device, such as a semiconductor laser device and a light emitting diode, which emits light having a wavelength of less than 700 nm and which utilizes the quantum effect in the light emitting region.
2. Description of the prior art:
In recent years, movement toward the development of an "information society" is rapid, and particularly conspicuous has been development of optical information processing techniques such as optical communication, optical discs, etc., which are based on optical devices incorporating semiconductor laser devices and light-emitting diodes. In these circumstances, optical devices which emit light having a wavelength in the visible region are required, and particularly high expectations exist for visible semiconductor laser devices. At the present, GaAlAs system semiconductor laser devices having an oscillation wavelength of 780 nm are in practical use as light sources for compact discs and video discs. In order to be able to handle a greater amount of information, it is necessary for the diameter of the focused beams to be decreased, and accordingly, semiconductor laser devices generating a shorter wavelength of light are required.
As a semiconductor material with an energy gap that corresponds to this region, (Al.sub.x Ga.sub.1-x).sub.y In.sub.1-y P, the lattice constant of which is matched to the lattice constant of a GaAs substrate, is receiving some attention. Because there are some difficulties with the growth of this material in the current liquid-phase epitaxial method, the newest methods available, molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MO-CVD) using organometallic compounds, are being much studied for application with this material. With MBE and MO-CVD, growth occurs depending upon the transport limit. The (Al.sub.x Ga.sub.1-x).sub.y In.sub.1-y P system contains three elements in Group 3 of the periodic table, so that if the proportions y of In, Al, and Ga are changed from the fixed value, the mis-match of the lattice thereof to that of the GaAs substrate becomes noticeable, and crystals of good quality cannot be obtained. Therefore, it is necessary to control the proportions of Al, Ga, and In to within 0.1%, which calls for high accuracy. However, this is not necessarily easy if the considerations of distribution of this material within the wafer surface and reproducibility are added.
In addition, since it is possible by MBE and/or MO-CVD to have a growth speed of less than 1 .mu.m/h, it is feasible to control the thickness of thin single crystals. In particular, by MBE, control of the order of layers of monomolecules can be achieved. Thus, MBE and MO-CVD allow the production of the active region of semiconductor laser devices in extremely thin layers, less than 100 .ANG., thereby allowing for stepwise changes in the density of the carrier, and enabling the production of a quantum well (QW) laser with a lowered threshold. This quantum well laser can be manufactured using (Al.sub.x Ga.sub.1-x).sub.y In.sub.1-y P. For example, as shown in FIG. 3, on an n-GaAs substrate 1, an n-(Al.sub.0.6 Ga.sub.0.4).sub.0.5 In.sub.0.5 P cladding layer (the thickness thereof being 1 .mu.m) 2, a non-doped multi-quantum well (MQW) active layer 3, a p-(Al.sub.0.6 Ga.sub.0.4).sub.0.5 In.sub.0.5 P cladding layer (the thickness thereof being 1 .mu.m) 4 and a p-GaAs cap layer 5 are successively grown by MBE, followed by the formation of a SiO.sub.2 film 6 on the cap layer 5. A stripe 9 is formed on the SiO.sub.2 film 6 by photolithography, etc., and then ohmic contacts 7 and 8 are formed, resulting in an oxide film striped laser device. The MQW active layer 3 is composed of alternate layers consisting of, as shown in FIG. 4, four non-doped (Al.sub.0.15 Ga.sub.0.85).sub.0.5 In.sub.0.5 P well layers (the thickness thereof being 50 .ANG.) 11 and three non-doped (Al.sub.0.4 Ga.sub.0.6).sub.0.5 In.sub.0.5 P barrier layers (the thickness thereof being 100 .ANG.) 12. In a QW laser with these components, in order for the lattice thereof to almost completely match that of GaAs, the semiconductors of the cladding layers 2 and 4, of the well layer 11, and of the barrier layer 12 must all be mixed crystals. Therefore, in the QW laser, it is necessary to confine the carriers by a well-shaped potential made in well layer 11. When mixed crystals are used, the chemical compounds (AlP, GaP and InP) that are the basis of the mixed crystals should be randomly mixed, resulting in fluctuations in the potential in the direction within the surface of the interface making up the quantum well, so that an ideal quantum well can not be achieved causing decreased operation efficiency. Moreover, if the well layer 11 and the barrier layer 12 are thin, this phenomenon will be more noticeable, and the problem of fluctuation resulting from mixed crystals within the well layer 11 and the barrier layer 12 will occur.